This invention relates to an OFDM receiving method and apparatus. More particularly, the invention relates to an OFDM receiving method and apparatus for receiving a signal multiplexed according to Orthogonal Frequency Division Multiplexing (OFDM) and applying FFT processing to the received signal to demodulate transmit data.
In wideband wireless communications, frequency-selective fading due to multipath causes a decline in channel quality. Multicarrier transmission is known as a method of modulation that is highly resistant to multipath fading. The method divides the transmission band into a plurality of (N) carriers (referred to as “subcarriers”), whereby a frequency diversity effect is obtained with respect to frequency-selective fading. This makes it possible to achieve high-quality wireless transmission. In FIG. 23, (a) is a diagram useful in describing a multicarrier transmission scheme. A serial/parallel converter 1 converts serial data to parallel data and inputs the parallel data to quadrature modulators 3a to 3d via low-pass filters 2a to 2d, respectively. In FIG. 23, the serial data is converted to parallel data comprising four symbols. Each symbol is a complex number and includes an in-phase component and a quadrature component. The quadrature modulators 3a to 3d subject each symbol to quadrature modulation by subcarriers having frequencies f1 to f4 illustrated in (b) of FIG. 23, a combiner 4 combines the quadrature-modulated signals and a transmitter (not shown) up-converts the combined signal to a high-frequency signal and then transmits the high-frequency signal. With the multicarrier transmission scheme, the frequencies are arranged, as shown in (b) of FIG. 23, in such a manner that the spectrums will not overlap in order to satisfy the orthogonality of the subcarriers.
Orthogonal Frequency Division Multiplexing (OFDM) is one mode of multicarrier transmission, in which frequency spacing is arranged so as to null the correlation between a modulation band signal transmitted by an nth subcarrier of multicarrier transmission and a modulation band signal transmitted by an (n+1)th subcarrier. In FIG. 24, (a) is a block diagram of an apparatus on the transmitting side based upon the OFDM scheme. The apparatus includes a serial/parallel converter 5 for converting serial data (in)to parallel data comprising a plurality of (e.g., N) symbols (I+jQ, which is a complex number). An IFFT (Inverse Fast Fourier Transform) arithmetic unit 6, which is for the purpose of transmitting the symbols as subcarriers having a frequency spacing shown in (b) of FIG. 24, applies an inverse fast Fourier transform to the frequency data to effect a conversion to a time signal in which subcarrier frequency components have been multiplexed, and inputs the real and imaginary parts to a quadrature modulator 8 through low-pass filters 7a, 7b. The quadrature modulator 8 subjects the input data to quadrature modulation, and a transmitter (not shown) up-converts the modulated signal to a high-frequency signal. On the receiving side, N symbols (OFDM symbols) that have been transmitted by N subcarriers are demodulated and output by an operation that is the reverse of the operation performed on the transmitting side (i.e., by a time-to-frequency conversion using FFT).
In accordance with OFDM, a frequency assignment of the kind shown in FIG. 24(b) becomes possible, thereby enabling an improvement in the spectrum efficiency. OFDM is different from other multicarrier transmission schemes that modulate their carriers independently, and since modulation/demodulation is performed at a stroke by an FFT, an orthogonal relationship is established among the carriers. Further, by adding on a guard interval signal on the transmitting side, it is possible to eliminate inter-symbol interference (ISI) caused by multipath delay. FIG. 25 is a diagram for describing the insertion of a guard interval (GI). If an IFFT output signal conforming to one OFDM symbol is adopted as one unit, insertion of the guard interval signifies copying the tail-end portion of the signal to the leading end thereof.
Thus, with OFDM, multipath equalization basically is unnecessary. However, in order to avoid causing a decline in performance, a guard interval that is larger than the maximum delay time of multipath envisioned in the system must be set in such a manner that ISI will not occur. Though inserting the guard interval (GI) makes it possible to eliminate the influence of ISI caused by multipath, a tradeoff is involved in that the guard interval diminishes transmission efficiency at the same time.
In order to mitigate the decline in transmission efficiency, it is necessary to make the OFDM symbol duration as large as possible, i.e., to make the guard ratio {[guard interval (Tg)]/[OFDM symbol duration (Tu)]} as small as possible. From this viewpoint, the carrier spacing (Δf) in the given bandwidth should be made small, i.e., the number of carriers should be increased.
However, due to fading, the receive signal varies not only along the time direction but also along the frequency direction. (This variation is a Doppler shift). Doppler shift proportional to moving velocity (v) is produced in the range of maximum Doppler frequency. If the carrier spacing is small, this variation is greater than one carrier and carrier synchronization on the receiving side is difficult. Furthermore, due to multipath, each path follows uncorrelated fading. As a consequence, frequency-selective fading, in which the variation sustained differs depending upon the frequency, occurs and the performance at the receiver is degraded. The reason for this is that inter-carrier interference (ICI) occurs because frequency fluctuation is independent from carrier to carrier (or, more specifically, from carrier group to carrier group within the coherence bandwidth). In order to suppress the degradation of performance caused by ICI, it is necessary to make the carrier spacing as large as possible. Thus there is a tradeoff with regard to transmission efficiency.
Thus, a technique for suppressing ICI is essential in order to raise transmission efficiency and avoid causing the degradation of performance in given system parameters. In order to suppress ICI in a multipath environment, each path must be estimated strictly and the fluctuation components thereof must be equalized. There are two techniques conceivable for achieving this, namely (1) FFT preprocessing on the receiving side, and (2) FFT post-processing on the receiving side.
In (1) involving preprocessing, the constantly changing multipath environment must be estimated accurately, estimation units for the maximum number of paths envisioned must be provided in the receiver, and equalization thereof is necessary. Consequently, problems remain in terms of practicality. Furthermore, performances are degraded in a multipath environment in which the envisioned maximum number of paths is exceeded.
In (2) involving post-processing, the arrangement is such that equalization is performed on a per-carrier basis because processing is executed after the FFT. Accordingly, equalization processing must be executed with respect to all carriers into which the signal component of the local carrier to be estimated has leaked, and ideally it is required that the OFDM receiver that executes the FFT processing have (N−1)-number of taps. In order to implement this for all carriers, N×(N−1) calculations would be required.
Accordingly, on the assumption that the major part of the interference energy is made up of the neighboring carriers, the complexity can be suppressed by a whole-number multiple if equalization is performed taking M (<<N)-number of neighboring carriers as the object of equalization. However, since performance degradation occurs and the degree of this decline depends upon the reception environment, no assurance can be given to a communication channel that has undergone imperfect equalization processing.